If you are attempting to remove a user from the access list of a document, you might see this message:
<user name> is the 'Author' and will retain VESA rights to this item.
This means that the user you are trying to remove is linked to the Author value that has been assigned to that document. As long as that Author has been assigned to the document, the user cannot be removed from the access list or have their access reduced.
Contact your system administrator for more information or learn more about Linking an Author Value to a User.